The crystal structure of ganophyllite; monoclinic subcell.

Abstract

The crystal structure of monoclinic subcell (a′=5.534, b′=13.565, c′=25.09Å, β′=93.96°, I2⁄a) of ganophyllite has been determined and refined to an R of 0.078. The structure of true cell (a=3a′=16.60, b=2b′=27.13, c=2c′=50.18Å, β=β′=93.96°, A2⁄a) can be explained based on this average subcell structure. This mineral is a modified layer structure as suggested by Smith and Frondel (1968). The octahedral (sheet) layer is curved like a sine wave along the b-axis and is sandwiched by two tetrahedral layers, each composed of triple chains. The upper and lower chains sandwiching the octahedral layer are always at the outside of the curvature and the small tetrahedral layer is inside. Thus the layer misfit is elegantly minimized in this structure. The interlayer cation is eight coordinated to four oxygens or hydroxyls at the edges of the chains and to four water molecules. The ordering of interlayer cations and water molecules followed by the site preference of aluminum in the tetrahedra, and resultant slight changes in polyhedra of the subcell structure explain why ganophyllite has a large cell. The structural formula of ganophyllite determined from X-ray microprobe and structure analyses is (K, Na, Ca)x Mn8(Si, Al)12(O, OH)32(OH)4·nH2O, with x=1∼1.5, and n≥4; Z=24 in the true large cell. The average monoclinic subcell structure also can explain the structure of triclinic polytype recently found by Jefferson (1978).